Should I watch this one still having seen David Suchet's first?
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shareYes. That way you'll see it as it should have been made.
shareWhile the tone of Suchet's version is overly dour, and the religious stuff seems weirdly shoehorned in and wasn't in the book, I still enjoy that version of the story. It is still pretty well-made at the end of the day and its flaws do not disqualify it.
I'd apply the same line of thinking to the recent Branagh version, for the record. Imperfect, but still enjoyable.
You say the '74 version is the story "as it should've been made." I do enjoy the '74 version--in fact, I credit it for turning me onto Agatha Christie--but I think it also is imperfect and in a significant way: of all the actors I've ever seen play Poirot in a major adaptation, Finney is the one I like the least.